Without limiting the scope of the invention, its background is described in connection with optical fiber sensors and their applications.
Most optical fiber biochemical sensors are based on measuring the RI of bio/chemical liquids using various sensing schemes such as interferometry [1], fiber gratings [2-5], and specialty fibers [7-11]. Interferometer-based RI sensors usually consist of two arms of light; one arm of light is influenced by the external medium and thus serves as the sensing arm while the other arm of light is used as the reference. When these two arms are combined to generate an interference pattern, a change in the external RI alters the optical path length of the sensing arm and thus causes a shift in the interference pattern. Interferometric RI sensors often require a mechanism to split the incoming light into two arms, resulting in a more complicated sensor system. Two types of fiber gratings, i.e. Fiber Bragg Grating (FBG) and Long Period Fiber Grating (LPFG), are commonly exploited for RI measurement. RI changes are measured from the shifts of the transmission/reflectance spectra due to the influence of the external RI on the coupling conditions of the fiber gratings. Because an LPFG couples the light from the core mode to the cladding modes, its transmission spectrum is highly sensitive to the changes of the external RI [2]. In comparison, the FBG-based RI sensors are usually much less sensitive because the light is mainly confined within the fiber core region. In order to increase the sensitivity, the cladding surrounding a FBG is often etched or thinned [3-4]. RI sensors based on fiber grating are usually expensive because of the stringent grating fabrication processes. Specialty fibers such as tapered fiber [7-8], and D-shaped fiber [9], microstructured fiber [10], and cladding stripped fiber [11] have also been developed for biochemical sensing. These types of optical fiber biochemical sensors require accessing the evanescent field at the fiber core/cladding interface. As such, precision micromachining is required in order to remove a part of the fiber cladding. In addition, many of these RI sensors require a long interaction length of more than a few millimeters to achieve a high RI sensitivity better than or comparable to the reported RI sensors [1,3,5-8].
For example, United States Published Patent Application No. 20030112443 (Hjelme et al.) describes a chemical sensing probe that detects chemical contents based on the volume change or the refractive index change of the chemically sensitive sensing materials that fill a Fabry-Perot cavity. The sensor requires the chemically sensitive sensing materials to react with the chemical contents so that either its volume or refractive index is changed. The change in volume and/or refractive index gives a change in an optical path length through the probe which can be measured interferometrically.
United States Published Patent Application No. 20090074349 (Hjelme et al.) describes the fabrication of interferometric fiber optic probes employing hydrogel sensor material that is responsive to an analyte; and to probes produced thereby. The invention relates particularly to probes which are suitable for invasive measurements of analytes in a live body. The sensor is fabricated on one fiber while the UV light is delivered by a second fiber.
U.S. Pat. No. 5,277,872 (Bankert) describes an optical fiber pH microsensor that includes an optical fiber having a portion of the surface of a light conducting core covered with a layer of a pH sensitive dye material. The dye material is covalently bonded to a polymeric matrix which is in turn covalently bonded to the optical fiber core to prevent leaching of the indicator dye material during extended use. The dye material is crosslinked in situ over the tip of the optical fiber core to yield a hydrophilic, ion permeable pH sensor which can be used intravascularly to monitor blood pH.
In addition, optical fiber sensors have also been widely used for temperature sensing since they have many advantages than conventional temperature sensors, e.g., they can safely operate in strong electromagnetic fields, in explosive or chemically aggressive environment and at areas under high voltage [12]. Among various fiber optic sensors, Fabry-Perot interferometric (FPI) sensors have distinct advantages over the others such as compactness, high sensitivity, small size, and polarization independence. FPI-based fiber optic sensors can be grouped as extrinsic FPI (EFPI) sensors and intrinsic FPI (IFPI) sensors. For EFPI sensors, the light signal is delivered and collected by the optical fiber and the modulation of the light occurs outside of the fiber. While in the IFPI sensor, the modulation of the light takes place inside the fiber. As “all fiber” sensors, IFPI sensors can reduce or eliminate the bonding problems experienced with extrinsic sensors. IFPI sensors are also more versatile for installation and are more robust. On the other hand, IFPI sensors are usually more difficult and expensive to fabricate [13]. Most of the reported IFPI sensors are based on manufacturing thin-film mirrors on the cleaved fiber end-face through vacuum deposition, magnetron sputtering or electron-beam evaporation [14-15]. However, thin film mirrors can easily be damaged. Besides, it is difficult to control the film thickness and flatness with precision. Another method that is being used to manufacture IFPI sensors is splicing two fibers with different core diameters as a reflective mirror [16]. But in order to fabricate an IFPI sensor with this configuration, the ends of fibers have to be polished with four different polishing films beforehand to prevent large power losses, which is a long and tedious process [16].
There is, therefore, a need for an optical fiber sensor that provides improved sensitivity and does not require specialized or complex materials.